Study On Deep Foundation Pit Deformation Caused By Dewatering Of Confined Water

With the rapid development of utilization of urban underground space and urban rail transit construction,the excavation depth of foundation pit is more and more deep and the status of confined water treatment is of more importance in foundation engineering. A formula to determine the safety factor was presented firstly in the paper by using analytical calculation method. Then,the geology of foundation pit of subway station was detected by using ground penetrating radar,and the distribution of groundwater was confirmed. What’s more,the geologic parameters of confined aquifer were determined by using intelligent displacement back analysis after analyzing the geological survey report and field test data. Finally,It’s analyzed that the deep foundation pit deformation characteristic of dewatering of confined water under different flow features by using FLAC3 D. The main conclusions are as follow:(1)A formula to determine the safety factor when the plane and circular failure surfaces appear was presented by using analytical calculation method. The minimum depth of enclosure structure of the vertical pit slope in the study is 25.38 m.(2)The geological conditions of the subway station construction site were detected by using ground penetrating radar. The phenomenon of loose soil or Void was found at the measured area,And the groundwater appeared at 2m from the ground. Ground settlement displacement, Lateral displacement of diaphragm retaining walls and displacement of bottom heave were processed and analysed. The deformation of foundation pit was summarized.(3)The sensitivity of confined aquifer’s different parameters to foundation pit settlement was analyzed by using orthogonal numerical simulation test,and there were four parameters of the confined aquifer need to be inverted: the elastic modulus E, Poisson’s ratio,permeability coefficient K and porosity n. Back analysis of learning and testing samples was built by uniform test method to build,and the training function within the margin of error was obtained by studying and testing the samples through MATLAB neural network toolbox,and then the parameters values of confined aquifer were inverted according to the measured data. The forward test results showed that the maximum relative error is 7.89% between simulation values and measured values of scene, and that the high degree of agreement can meet the need of engineering precision.(4)According to the calculation formula of inrushing for confined water foundation pit,the author found that the water head of the confined aquifer of the station foundation pit must decrease 9m before excavation. The settlement of foundation pit presented "parabola" caused by dewatering of confined water. The maximum settlement appeared at a distance of 2 times the maximum depth of excavation. Settlement trend was similar to precipitation funnel shape, which proved that the settlement of soil out of the maximum settlement was caused by dewatering. Settlement deformation caused by dewatering of confined water mainly occurred in the overlying soil above the confined aquifer. Settlement deformation presented "small top and big bottom" feature, and the maximum displacement of soil occurred in the boundary between confined aquifer and overlying soil.(5)In the first type of flow characteristics, soil settlement deformation caused by dewatering of confined water was minimum. The second type of flow characteristics leaded to the maximum settlement deformation compared with the other condition. It was 141% higher than dewatering on foundation internal with the same enclosure protection project. In the third type of flow characteristics, with the increase of the depth of underground continuous wall inserted into the confined aquifer, the settlement of foundation decreased. Sidesway of underground continuous wall and upheaval in the bottom of deep foundation pit was mainly depended on the stiffness of support structures and depth of excavation rather than the influence of confined water. The maximum sidesway of underground continuous wall occurred near the bottom of deep foundation pit. Upheaval at the bottom of foundation pit presented the characteristics that the center is higher than the side.